1. The Field of the Invention
The present invention pertains to a method and apparatus for effecting cross-well seismic surveys and in particular, to a down-hole seismic source using piezoelectric means and a reaction mass for generating a seismic signal.
2. The Prior Art
There are many well-known methods and apparatus for effecting seismic surveys from the surface of the earth and even from the surface of water. All of these known surveying techniques basically involve a first surface station wherein a seismic signal is generated by an explosion, by a sudden emission of compressed gas or the like, or by the rapid descent of a significant mass against the earth, each providing an essentially single signal, or a mechanical vibrator providing generated signal which continuously sweeps a limited range, and at least one second station, remote from the first station, having at least one listening or receiving device to sense and record the reflected seismic wave which was generated at the first station. The recorded data from the second station is then processed to form a geological profile of the area. All of these known methods and apparatus have found widespread use and are effective to a degree. However, because of the fact that the upper surface layers of the earth have been weathered over the ages, they serve to greatly antennuate high frequency signals generated from the earth's surface. This antennuation is such that when the signal gets to the depths of many oil and gas reserves, it is substantially impossible to obtain any resolution for anomalies less than 100 feet in size. Thus conventional surface seismic surveying can cover very large areas, but with limited resolution.
In the technique known as well-logging, it is possible to get very accurate determination of the subsurface structure, but the devices of this nature are limited as to the distance from the borehole which can be surveyed. It is usually a matter of only inches and when one considers that the wells are drilled generally hundreds of feet, if not hundreds of yards apart, then it becomes quite apparent that this type of seismic surveying would miss large bodies of potential value.
A newly developing technology is known as cross-well seismic surveying and involves placing a seismic source in a primary well and receivers in at least one secondary well in the immediate vicinity of the first well. It has been possible to process the data recorded at the secondary wells to form high resolution images of the entire area between the wells. In cross-well seismic surveying, once both the source and receivers are lowered into the earth, the ability to pass high frequency seismic signals is greatly improved, particularly when the source is clamped to the borehole wall. Cross-well seismic surveying can provide resolution on the order of ten feet.
Since a borehole seismic source and borehole receiver assemblies both operate in the same environment as well-logging tools, they must share many of the same physical characteristics. Thus, each must be capable of being encapsulated, suspended from a cable or wire and lowered into the borehole, which is usually fluid filled, to considerable depths at which extremes of temperature and pressure may be encountered. There is also limited capacity for communication between the downhole tool and the surface as defined by the multi-conductor cable. It may be preferable to have the seismic source controlled by means of a downhole computer having instructions stored in its memory and for the receiver assemblies to have downhole recording capabilities.
At the present time there are few borehole seismic sources which are commercially available. One is a borehole vibrator that operates by hydraulic power supplied from the surface through coiled tubing. This tool is approximately 120 feet long and produces a powerful sine wave signal in the borehole fluid, which signal varies in frequency from 20 to 120 hertz. Another borehole seismic source is an air gun that is essentially the same as the well-known marine air gun. It is powered by compressed air supplied from the surface through a special umbilical cable containing a rubber hose. This device releases a high pressure localized burst of air into the borehole fluid.
The present invention is distinct from these previously known mechanical devices in that it is electronic in nature. The present invention incorporates at lease one piezoelectric bender bar assembly which can be energized from the surface by a high power amplifier. This device, which is somewhat similar to naval sonar transducers, can operate over a standard multi-conductor wire line cable. The bender bar assembly, as an example, can be constructed from sets of ceramic piezoelectric crystals bonded to the two sides of a thin metal plate to form an assembly, typically about 3 inches wide and 24 inches long. Both sets of crystals are energized simultaneously, by applying a high voltage with opposite polarity, causing one crystal to expand while the other contracts. This results in the assembly, since both ends of the bar are fixedly clamped, bending in the middle portion. Although there are several resonant points, the bender bar motion generally follows the applied electrical signal wave form.
When a piezoelectric crystal bender bar assembly is used as a borehole seismic source, the assembly is preferably enclosed in a oil filled flexible tube. Pressure waves generated by bender bar motion couple through the flexible tube and through the borehole fluid. Some of the energy is coupled from the borehole fluid through the metal well casing into the earth. The seismic signal radiating from the borehole is a horizontally traveling pressure or P wave. Although it is not well understood, the radiation pattern in earth is believed to be nominally symmetrical about the well bore axis.
The seismic wave generated by the subject invention will have pressure (P) and shear (S) wave components. The P wave will arrive at the receiver first as the S wave travels at only about half the velocity of a P wave. The S wave is affected only by rock structure and not by liquid contact while the P wave is affected by liquid contact. The ratio of the P wave to the S wave can be used to estimate porosity of the rock through which the waves pass.
It has been reported that conventional devices have been able to transmit seismic signals of up to 3,000 hertz through limestone between cased wells 1,500 feet apart.
With any seismic source that couples energy to the earth by pressurizing the borehole fluid, most of the energy (90 to 95%) remains trapped inside the borehole fluid in the form of tube waves, which are pressure waves that travel vertically up and down the length of the fluid filled casing. Not only is this inefficient, but the tube wave is a major cause of unwanted signals in the form of background noise. The tube waves are strongly reflected from the bottom of the hole and from the top of the fluid level and to a lesser extent from any anomaly in the casing. Each of these points causes a fraction of the tube wave energy to be transmitted into the earth and that effectively becomes another seismic source point.